You have entered the website of the Water and Environmental Research Institute of the Western Pacific (WERI) at the University of Guam. We are currently six faculty and five staff and our mission is to seek solutions through research, teaching and outreach programs, to issues and problems associated with the location, production, distribution and management of freshwater resources in Guam, the CNMI and the FSM.
The long-term variations of rainfall on Saipan are very similar to those on Guam. As on Guam, the distribution of rainfall on the island is affected by the topography, and the mean annual rainfall totals among recording stations on Saipan differ by as much as 15 inches (380 mm), or approximately 20%. The region in the vicinity of Saipan's International Airport receives the lowest annual total of about 75 inches (1900 mm). The highest measured annual average of approximately 90 inches (2300 mm) occurs at Capitol Hill, and extends along the high ground from Marpi to Mount Tagpochau.
The causes of extreme rainfall events on Saipan are typhoons, monsoon squall lines, and other so-called mesoscale weather systems. Unlike the spatial distribution of mean annual rainfall, the spatial distribution of short-period extreme rainfall is independent of the island topography. The highest-intensity rainfall events are caused by typhoons. This may be true for all intervals, from the peak 15-minute rainfall to the peak 24-hour rainfall. Because of typhoons, the probability distribution of 24-hour rainfall events is mixed (i.e., without typhoons, the return-periods for daily rainfall in excess of 10 inches would be much longer).
More rainfall on Saipan occurs in the 12-hour span between midnight and noon than in the 12-hour span between noon and midnight; with an absolute minimum in the evening. This is the typical rainfall distribution over the open ocean undisturbed by the effects of island topography and land-surface heating, and is caused by diurnally varying radiative processes in the tropical oceanic atmosphere. Unlike larger or more mountainous islands, Saipan is too small to appreciably alter the over-water diurnal rainfall pattern.
Once thought to be largely random, rainfall in the tropics was found to have a strong month-to-month variation caused by a phenomenon known as the Madden-Julian Oscillation (MJO). The rainfall on Saipan is probably affected by the MJO. The manifestation of the MJO signal at Saipan is to produce several weeks of wet weather broken by a week or two of hot dry weather. The signal is not always very strong, but during some years (such as during the rainy season of 2004), it is particularly strong.
Inter-annual variations of Saipan's rainfall are closely linked to the El Niño/Southern Oscillation (ENSO) phenomenon. Saipan is in an ENSO core region that features very dry conditions in the year following El Niño, and an increase in the level of threat from typhoons during an El Niño year. Large inter-decadal variations in rainfall and also in the distribution of typhoons are noted. The causes of these remain unknown.
